1. Field of the Invention
This invention relates to polyorganosiloxane compositions. More particularly, this invention relates to polyorganosiloxane compositions that cure to elastomeric materials capable of adhering to inorganic substrates such as metals, glass and ceramics.
2. Description of the Prior Art
One class of polyorganosiloxane compositions cure by a platinum-catalyzed hydrosilation reaction to form elastomeric materials referred to in the art as "silicone rubber". The curable compositions typically comprise a polyorganosiloxane containing at least two vinyl or other ethylenically unsaturated hydrocarbon radicals per molecule, an organohydrogensiloxane containing an average of at least three silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst. Depending upon the specific reactants present in the composition curing can occur at ambient or elevated temperatures.
A second class of polyorganosiloxane compositions cure at room temperature, typically about 25 degrees centigrade, in the presence of moisture. The reactants in this type of composition comprise a hydroxyl terminated polydiorganosiloxane and a silane or other relatively low molecular weight organosilicon compound containing at least three alkoxy or other hydrolyzable groups per molecule. Alternatively, these two materials can be pre-reacted to form a polydiorganosiloxane containing at least two hydrolyzable groups at each end of the molecule. Irrespective of the manner in which it is prepared, the composition reacts in the presence of moisture and a suitable catalyst for the hydrolysis reaction to form a crosslinked material. Suitable catalysts include metal compounds such as inorganic and organic tin compounds and titanium compounds such as orthoesters and chelates formed by reacting these orthoesters with polyfunctional, enolizable ketones.
The unique physical and chemical properties of cured polyorganosiloxane elastomers, particularly their durability and resistance to solubilization and degradation under a variety of conditions make these elastomers desirable for numerous applications, including protective coatings for substrates such as semiconductor devices that are susceptible to damage by physical abuse or by exposure to humidity and/or contaminants present in the environment.
Polyorganosiloxane compositions that cure by a platinum catalyzed hydrosilation reaction are useful for those applications where a curing time of hours or even minutes is required at ambient or elevated temperatures. A deficiency of many polyorganosiloxane elastomers of this type is their relatively poor adhesion to metals and other inorganic substrates.
Moisture curable polyorganosiloxane compositions are typically one part materials that cure at room temperature. The compositions must be allowed to cure for at least several hours even at elevated temperatures to achieve a tack-free surface that is not sticky to the touch. Curing times of 24 hours or longer are usually required to achieve a useful level of physical properties.
One method for improving the adhesion of polyorganosiloxane elastomers to inorganic substrates such as metal and glass is to first coat the substrate with a liquid primer composition that typically includes at least one moisture reactive organosilicon compound. U.S. Pat. No. 4,401,500, which issued to Hamada and Yasuda on Aug. 30, 1983 teaches using an alkyltrialkoxysilane, a organic hydroperoxide and an organic solvent as a primer composition for bonding a silicone rubber to a substrate.
Primer compositions containing a vinyl substituted silane, an organohydrogensiloxane, a platinum-based catalyst and an organic titanate are disclosed in Japanese patent publication No. 84/220349, which was published on Dec. 11, 1984.
Primers are less than desirable as a means to achieve bonding between a polyorganosiloxane elastomer and a substrate because the primer composition must be applied and allowed to cure or at least dry before the curable polyorganosiloxane elastomer composition can be applied to the substrate.
A second method for improving the bonding between a polyorganosiloxane elastomer curable by a platinum catalyzed hydrosilation reaction and a substrate is by the use of adhesion promoters that are added to the curable polyorganosiloxane composition. U.S. Pat. No. 4,196,273, which issued to Imai and Tanaka on Apr. 1, 1980 teaches adding an alkoxysilane having an aliphatically unsaturated hydrocarbon radical to a polyorganosiloxane composition that is curable by a platinum catalyzed hydrosilation reaction.
U.S. Pat. No. 4,311 739, which issued to Hardman and Dujak on Jan. 19, 1982 teaches the use of partial hydrolysis products of an aliphatically unsaturated hydrolyzable alkoxysilane such as vinyltriethoxysilane to improve the adhesion exhibited by the same type of polyorganosiloxane compositions described by Imai et al.
Adhesion promoters consisting essentially of the combination of (1) a polyorganosiloxane containing a hydroxyl group and a vinyl radical and (2) an epoxysilane are disclosed in U.S. Pat. No. 4,087,585, which issued to Schulz on May 2, 1978.
Certain moisture cured polyorganosiloxane elastomers that in the uncured form comprise a mixture or reaction product of a liquid hydroxyl terminated polydiorganosiloxane and a tri or tetraalkoxysilane in addition to an organotitanate curing catalyst have been shown to function as protective coatings for integrated circuits and other semiconductor devices. The disadvantage of these compositions is their relatively long cure times.
The present inventor found that when up to about ten percent by weight of a moisture curable composition of this type is blended with a platinum-catalyzed polyorganosiloxane composition of the type described hereinbefore, the resultant elastomer combines the adhesion of the moisture cured elastomer with the rapid curing rate and good dielectric properties of the platinum catalyzed composition.